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Jul 08, 2001, 02:32 AM
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Mikey_Flies's Avatar

Small Battery Questions

How much difference would 4 mOhms of resistance make to a power pack for a small model. In particular I'm thinking of a model drawing between 2 and 3.5 amps and comparing either 8 110 nicads, or 8 120 TA Nicads. The 110's are listed as 30 mOhm resistance at 50% discharge, the 120's at 34 mOhm at 50% discharge. Now, will this mean that the 120's can only deliver ~88% of the current that the 110's can? (30/34, as v=ir -> i=r/v and v is the same thus i2=r1/r2 * i1) It looks like I can save 8 grams by going to the 120's vs the 110's (Doesn't sound like much, but a 110 cell weighs 7 gm and a 120 weighs 6, so that is really saving the weight of a whole cell!!) but I don't want to lose a lot of the 110's power.
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Jul 08, 2001, 03:46 AM
Registered User
It's an interesting question but unfortunately you can't simply apply Ohms law to the effective resistance of a cell and get accurate answers. It only works over a limited range of loads. The only way to really find out is to try it.

The problem is that we are abusing these cells by dragging way too much current out of them so we are outside the manufacturers specs almost all of the time.

Jul 08, 2001, 04:01 AM
Registered User
Mikey_Flies's Avatar
Ya :/ at 3.5 amps, I'm exceeding 1C by aprox 32 times with a 110 cell. Fortunately this doesn't happen for long (although at 2 it is still 19 x 1C) I think that the weight savings will offset the thrust loss. If not, I can always add another cell and still be 2 grams ahead I suppose I'm not sure if the motor can take 9 cells for very long though :/ and field charging will be right out (8 you have to cheat a little)